1. Field of the Invention
The present invention relates to a base station device, and particularly to a base station device provided with a plurality of antennas.
2. Description of the Background Art
Cellular phone systems and handy cellular phone systems are now in widespread use among many users. In these systems, terminal devices perform predetermined data communication over channels allocated by a base station device. One base station device allocates a plurality of channels to terminal devices, and multiplexes these terminals. Particularly, the cellular phone systems and handy cellular phone systems generally use a TDMA (Time Division Multiple Access) system as a multiplexing technique. According to the TDMA system, a time axis is divided into predetermined time periods called “time slots”, and these times slots are allocated to the respective users for multiplexing.
An antenna directivity control technique based on an adaptive array antenna has been studied as a technique for achieving stable connection with the terminal devices.
More specifically, an adaptive array base station device provided with antennas of n in number is configured such that phases and amplitudes of transmission/reception circuits of n in number are operated as follows. In the reception operation, a suitable phase difference and a suitable amplitude ratio are applied to a reception signal of each antenna for combination so that a reception gain is increased selectively in a desired direction. In the transmission operation, these suitable phase difference and amplitude ratio are applied to a transmission signal for each antenna so that a transmission gain is increased selectively in this desired direction.
Thereby, it is possible to improve gains of the reception and transmission with respect to the terminal device requiring the communication so that more stable communication can be achieved.
A technique for achieving effective use of frequencies according to the antenna directivity control technique has also been proposed (Japanese Patent No. 3326416).
According to this technique, the base station device with the plurality of antennas can attain two modes, i.e., an all-antenna mode and a group mode. In the all-antenna mode, all the antennas are used to form an array antenna pattern. In the group mode, the plurality of antennas are divided into a plurality of groups each including two or more antennas, and different frequencies are used for these groups to form individual array antenna patterns, respectively. Switching is done between these modes, and thus between use of one wave and use of multiple waves in accordance with the number of connections by the users. Thereby, frequency resources are efficiently used, and more users can be accommodated according to the above technique.
Under these circumstances, the inventors have recognized the following disadvantages.
For example, when all antennas are used to form an array antenna pattern in a technique disclosed in Japanese Patent No. 3326416, a plurality of local synthesizers are set to the same frequency. However, if hardware does not have completely the same characteristics, differences in characteristic affect characteristics of the adaptive array. Thereby, interference removal is not performed so that transmission/reception characteristics deteriorate.